.\"	$NetBSD: time.9,v 1.1 1995/11/25 21:24:53 perry Exp $
.\"
.\" Copyright (c) 1994 Christopher G. Demetriou
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"      This product includes software developed by Christopher G. Demetriou
.\"      for the NetBSD Project.
.\" 3. The name of the author may not be used to endorse or promote products
.\"    derived from this software without specific prior written permission
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd May 4, 2021
.Dt TIME 9
.Os
.Sh NAME
.Nm boottime ,
.Nm time_second ,
.Nm time_uptime
.Nd system time variables
.Sh SYNOPSIS
.In sys/time.h
.Pp
.Vt extern struct timeval boottime ;
.Vt extern time_t time_second ;
.Vt extern time_t time_uptime ;
.Sh DESCRIPTION
The
.Va boottime
variable holds the estimated system boot time.
This time is initially set when the system boots, either from the RTC, or from a
time estimated from the system's root filesystem.
When the current system time is set, stepped by
.Xr ntpd 8 ,
or a new time is read from the RTC as the system resumes,
.Va boottime
is recomputed as new_time - uptime.
The
.Xr sysctl 8
.Va kern.boottime
returns this value.
.Pp
The
.Va time_second
variable is the system's
.Dq wall time
clock to the second.
.Pp
The
.Va time_uptime
variable is the number of seconds since boot.
.Pp
The
.Xr bintime 9 ,
.Xr getbintime 9 ,
.Xr microtime 9 ,
.Xr getmicrotime 9 ,
.Xr nanotime 9 ,
and
.Xr getnanotime 9
functions can be used to get the current time more accurately and in an
atomic manner.
Similarly, the
.Xr binuptime 9 ,
.Xr getbinuptime 9 ,
.Xr microuptime 9 ,
.Xr getmicrouptime 9 ,
.Xr nanouptime 9 ,
and
.Xr getnanouptime 9
functions can be used to get the time elapse since boot more accurately
and in an atomic manner.
The
.Va boottime
variable may be read and written without special precautions.
It is adjusted when the phase of the system time changes.
.Sh SEE ALSO
.Xr clock_settime 2 ,
.Xr ntp_adjtime 2 ,
.Xr settimeofday 2 ,
.Xr bintime 9 ,
.Xr binuptime 9 ,
.Xr getbintime 9 ,
.Xr getbinuptime 9 ,
.Xr getmicrotime 9 ,
.Xr getmicrouptime 9 ,
.Xr getnanotime 9 ,
.Xr getnanouptime 9 ,
.Xr microtime 9 ,
.Xr microuptime 9 ,
.Xr nanotime 9 ,
.Xr nanouptime 9
.Rs
.%A "Poul-Henning Kamp"
.%T "Timecounters: Efficient and precise timekeeping in SMP kernels"
.%J "Proceedings of EuroBSDCon 2002, Amsterdam"
.%O /usr/share/doc/papers/timecounter.ascii.gz
.Re
.Rs
.%A "Marshall Kirk McKusick"
.%A "George V. Neville-Neil"
.%B "The Design and Implementation of the FreeBSD Operating System"
.%D "July 2004"
.%I "Addison-Wesley"
.%P "57-61,65-66"
.Re
